A 6-month-old boy presented at the Ocular Oncology Service, Wills Eye Hospital, with leukocoria and a right-eye esotropia. Examination revealed an exophytic retinal mass in the right eye, measuring 14 × 12 × 7 mm, with shallow subretinal fluid and subretinal seeding (A). The left eye was normal. The patient was diagnosed with unilateral retinoblastoma, characterized as International Classification of Retinoblastoma group D, and treated with three cycles of intra-arterial chemotherapy (IAC) using melphalan 5 mg. Genetic testing revealed a rare, poorly characterized, c718+5G>T germline mutation in exon 7 of the RB1 gene. There was no family history of retinoblastoma, but family genetic testing demonstrated similar germline mutation in the father, paternal grandmother, and sibling, all with no evidence of intraocular tumor, suggesting low penetrance of this genetic variant. The tumor showed complete response to IAC, with regression to a calcified scar of 3.4 mm in thickness and resolution of subretinal fluid and seeds (B). At 2 years’ follow-up, there was no recurrence, and visual acuity was fix-and-follow.